This invention relates to multimode optical fibers and more particularly to multimode optical fibers wherein one or more dopants are radially graded throughout the core of the fiber in order to minimize the modal dispersion.
In the disclosure entitled "Ternary Fiber Glass Composition for Minimum Modal Dispersion Over a Range of Wavelengths" presented by I. P. Kaminow and H. M. Presby at the Optical Fiber Transmission II Conference, Feb. 22, through Feb. 24, 1977 at Williamsburg, Virginia, an optical fiber was described wherein phosphorus pentoxide and germanium dioxide are radially graded throughout the core of the optical fiber in order to obtain minimum modal dispersion over a broader range of wavelengths. An optical fiber of this type is also described in a copending patent application by Kaminow and Presby, Ser. No. 731,786, entitled "Silica Based Optical Fiber Waveguide Using Phosphorus Pentoxide and Germanium Dioxide", and filed on Oct. 12, 1976.
In the fiber disclosed by Kaminow and Presby, phosphorus pentoxide is caused to appear with maximum concentration on the axis of the core and this concentration is radially graded to zero at the core-cladding interface. The germanium dioxide on the other hand is caused to appear with zero molar concentration on the axis of the core and its concentration is radially graded to a maximum at the core-cladding interface. To avoid any step or discontinuity in the refractive index at the core-cladding interface, the fiber must also have a constant level of germanium dioxide a predetermined distance into the cladding of the fiber.
One of the more desirable techniques of manufacturing these type fibers is the modified chemical vapor deposition (MCVD) process. See the article entitled "A New Technique for the Preparation of Low-Loss and Graded-Index Optical Fibers", by J. B. MacChesney, P. B. O'Connor, and H. M. Presby, Proceedings of IEEE, Vol. 62 No. 9, pages 1280-1281, September, 1974. In order to provide a constant level of germanium dioxide in the cladding of the fiber, the silica tube utilized as a preform in the MCVD process would serve only as support inasmuch as a cladding layer with germanium dioxide must first be deposited on the wall of the tube. Only then can the dopants be radially graded in order to manufacture that part of the preform which serves as the core of the optical fiber. As a result, the fiber would be larger in cross-section than a fiber wherein the silica tube could serve as the cladding. A larger fiber is obviously less desirable since fewer fibers can then be utilized in the make up of any given cable diameter.
If the germanium dioxide is not continued into the cladding, the silica tube can serve as the cladding and a smaller fiber will necessarily result, but this fiber will have a step or discontinuity in the refractive index at the core-cladding interface. This step in the refractive index at the core-cladding interface will primarily effect the higher order modes in the waveguide. In fiber waveguides with a significant level of imperfections, these higher order modes will have larger radiation losses and as a result, have very little effect on the dispersion of the output pulse. If, however, the imperfections are kept to a minimum and a relatively good waveguide is manufactured, the higher order modes interact with this step at the core-cladding interface and arrive at the receiving end at a time which is significantly different from the arrival of the lower order modes, thereby resulting in increased pulse dispersion. Any filtering which could be performed to eliminate the higher order modes simply decreases the amount of energy received by the detector thereby adding to the undersirable attenuation of the system.